Ammonia is a compound that is highly important for industry, being not only used as a starting material for production of nitrogen fertilizers, nitric acid and urea, but also as a refrigerant for refrigerating machines, as a solvent, as metal refining material, and the like.
The Haber-Bosch process is generally used for production of ammonia, and normally synthesis is accomplished from nitrogen and hydrogen using an iron-based catalyst at a pressure of several hundred atmospheres and a temperature of 400° C. to 500° C.
The hydrogen used for production of ammonia is usually produced by steam reforming of hydrocarbon fuel, represented by the following formulas (A1) and (A2).CnHm+nH2O→nCO+(n+m/2)H2  (A1)CO+H2O→CO2+H2  (A2)CnHm+2nH2O→nCO2+(2n+m/2)H2  Overall reaction:
Thus, carbon dioxide is usually generated when producing hydrogen to be used for production of ammonia.
In recent years, however, it has become a major worldwide goal to limit generation of carbon dioxide, in consideration of problems such as global warming.
In this regard, there have been proposed methods for producing hydrogen without using hydrocarbon fuel, wherein thermal energy such as solar thermal energy or nuclear thermal energy is used to split water into hydrogen and oxygen (PATENT DOCUMENT 1, Non Patent Document 1).
As a method for producing hydrogen from water using thermal energy, there has been proposed a method known as the S—I (sulfur-iodine) cycle method, represented by the following formulas (B1) to (B3):H2SO4(liquid)→H2O(gas)+SO2(gas)+½O2(gas)  (B1)
(Reaction temperature=about 950° C., ΔH=188.8 kJ/mol-H2)I2(liquid)+SO2(gas)+2H2O(liquid)→2HI(liquid)+H2SO4(liquid)  (B2)
(Reaction temperature=about 130° C., ΔH=−31.8 kJ/mol-H2)2HI(liquid)→H2(gas)+I2(gas)  (B3)
(Reaction temperature=about 400° C., ΔH=146.3 kJ/mol-H2)
The overall reaction in the S—I (sulfur-iodine) cycle method represented by formulas (B1) to (B3) above is as follows:H2O→H2+½O2 
(ΔH=286.5 kJ/mol-H2 (based on higher calorific value)
(ΔH=241.5 kJ/mol-H2 (based on lower calorific value)
The reaction (B1) above can be divided into two elementary reactions of the following formulas (B1-1) and (B1-2):H2SO4(liquid)→H2O(gas)+SO3(gas)  (B1-1)
(Reaction temperature=about 300° C., ΔH=90.9 kJ/mol-H2)SO3(gas)→SO2(gas)+½O2(gas)  (B1-2)
(Reaction temperature=about 950° C., ΔH=97.9 kJ/mol-H2)
In other words, producing hydrogen by the S—I cycle method requires the highest temperature for the sulfur trioxide (SO3) decomposition reaction (B1-2), and it is not easy to obtain the high temperature required for this reaction.
For this problem, in Non Patent Document 1, solar thermal energy is used as a heat source, while natural gas is combusted, as necessary, to provide additional thermal energy.
It has also been proposed to use a platinum catalyst to lower the temperature required for the sulfur trioxide decomposition reaction (B1-2). However, it is known that when a platinum catalyst is used in this reaction, despite excellent properties being exhibited when the catalyst begins to be used, the oxygen generated by the reaction oxidizes the platinum, and forms coarse platinum particles, that result in lower catalytic activity. Also, because platinum catalysts are expensive, they are difficult to use on an industrial scale.
In this regard, Non Patent Document 2 proposes using a catalyst selected from the group consisting of platinum (Pt), chromium (Cr), iron (Fe) and their oxides, supported on an alumina support, in order to lower the temperature required for the sulfur trioxide decomposition reaction.
For the S—I cycle method, Patent Document 2 proposes, for the reaction (B2) above, i.e. for the reaction of obtaining hydrogen iodide and sulfuric acid from iodine, sulfur dioxide and water, conducting reaction between sulfur dioxide and water on the positive electrode side of a cation exchange membrane, and conducting reaction of iodine on the negative electrode side of a cation exchange membrane, to eliminate the subsequent separation procedure.
In addition to the S—I cycle method, other methods of splitting water into hydrogen and oxygen utilizing thermal energy include the Westinghouse cycle, the Ispra-Mark 13 cycle method and the Los Alamos Science Laboratory cycle method, but these methods also require decomposition of sulfur trioxide into sulfur dioxide and oxygen, as in formula (B1-2).